2000 Volume 18 Issue 3 Pages 449-458
Changes in microstructures of modified HP-type heat-resistant cast alloys with long term aging were investigated using the as cast, artificially aged and practically used alloys at the elevated temperature. The microscopic observation and the X -ray diffraction analysis for microconstituents in heat-treated alloys were conducted in order to clarify the changes in morphologies and phases of microconstituents with heat treatment. In the niobium containing alloys, chromium-rich carbide of M7C3 and niobium carbide (NbC) changed to M23C6 and nickel-niobium silicide (7 phase) with long term aging, respectively. On the other hand, only chromium-rich carbide (M7C3 or M23C6) precipitated in as cast condition in the niobium-free alloys and the transformation of M7C3→ M23C6 or M23C6+nickel-rich silicide (G phase) occurred in the long term aging situation. The lamellar structure of eutectic microconstituents in the as cast alloy tended to agglomerate and coarsen with aging, and finally linked forming a continuous network surrounding the cellular dendrite boundaries. Zr addition to the HP cast alloy made it possible to reduce the microconstituents on the dendrite boundary due to the refinement of dendritic cell.